Communications
DOI: 10.1002/anie.200703123
Radialenes
Planar [6]Radialenes: Structure, Synthesis, and Aromaticity of
Benzotriselenophene and Benzotrithiophene**
Asit Patra, Yair H. Wijsboom, Linda J. W. Shimon, and Michael Bendikov*
Radialenes (e.g. [6]radialene, 1) are a relatively modern class
of alicyclic hydrocarbons in which all carbon atoms in the ring
are sp2 hybridized and there are as many exocy-
Herein, we report the efficient
synthesis, X-ray crystal structures,
and characterization of previously
clic double bonds as possible.[1,2] The first radia-
unknown
c’’]triselenophene (5) and its
known[14]
sulfur analogue,
benzo[1,2-c:3,4-c’:5,6-
lene, 7,8,9,10,11,12-hexamethyl[6]radialene, was
prepared by Hopff and Wick in 1961.[3] Radia-
lenes and their derivatives have been the subject
benzo[1,2-c:3,4-c’:5,6-c’’]trithio-
of significant theoretical and experimental inter-
phene (4). Two other important
est.[1,2,4,5] New approaches to the synthesis of
compounds,
the
previously
radialenes have been developed, and these struc-
unknown selenophene-fused [8]annulene 7 and its known
sulfur analogue 6[15] were also synthesized as side products.[16]
Both 4 and 5 are rare examples of planar [6]radialenes. They
display an unusual geometry, electronic structure, and lack of
aromaticity in the planar six-membered ring.
We prepared compound 4 in one step from commercially
available 3,4-dibromothiophene (Scheme 1).[17] The coupling
of 3,4-dibromothiophene with [Ni(cod)2] in the presence of
1,5-cyclooctadiene (cod) and the neutral ligand Ph3P in dry
acetonitrile produced compounds 4 and 6 in a 1:1 ratio
tures have become important building blocks for organic
conductors and ferromagnets.[1,2,6] Radialenes are also of
considerable interest because of their exocyclic double bonds,
aromaticity, and conjugation.[1,2,4a] The parent [3]-, [4]-, [5]-,
and [6]radialenes polymerize at room temperature and are
unstable in air; however, many stable derivatives are
known.[1,2] It has been found experimentally that [3]-[7] and
[4]radialenes[8] usually have a planar structure.[9] [5]Radia-
lenes, such as decamethyl[5]radialene, usually have a twisted-
envelope or half-chair conformation.[10] In contrast to [3]-,
[4]-, and [5]radialenes, [6]radialenes (of type 1) have a chair
conformation, according to both experimental and theoretical
studies.[1,2,11] Perylene and triphenylene may not be consid-
ered as radialenes, because the p electrons are delocalized
over an aromatic sextet.[2] On the other hand, the condensed-
thiophene-fused compounds 2[12] and 3,[13] which are isoelec-
tronic with perylene, belong to the [6]radialene family.[2] To
the best of our knowledge, the only known example of a
planar [6]radialene is compound 2.[12]
Scheme 1. Preparation of compounds 4–7 by nickel-catalyzed cyclo-
oligomerization. DMF=N,N-dimethylformamide.
[*] Dr. A. Patra, Y. H. Wijsboom, Dr. M. Bendikov
Department of Organic Chemistry
Weizmann Institute of Science
(Table 1).[17,18] Compounds 4 and 6 are white solids that are
moderately soluble in organic solvents, such as hexane,
CHCl3, and dichloromethane. The same products were
obtained, but in a different ratio, when 2,2’-bipyridyl was
used as the neutral ligand (Table 1). To prepare 5 and 7 (the
previously unknown selenium analogues of 4 and 6, respec-
tively), 3,4-dibromoselenophene[19] was treated similarly with
Rehovot 76100 (Israel)
Fax: (+972)8934-4142
E-mail: michael.bendikov@weizmann.ac.il
Homepage:
Dr. L. J. W. Shimon
Chemical Research Support Unit
Weizmann Institute of Science
[**] We thank Prof. Amnon Stanger (Technion) and Prof. D. F.
Perepichka (McGill University) for helpful discussions. This
research is supported by the MINERVA Foundation. M.B. is the
incumbent of the Recanati Career Development Chair and a
member ad personam of the Lise Meitner-Minerva Center for
Computational Quantum Chemistry.
Table 1: Yields of 4–7 from the reaction in Scheme 1.
X
Ligand
Yield of 4 (X=S)
or 5 (X=Se) [%][a]
Yield of 6 (X=S)
or 7 (X=Se) [%][a]
S
S
Se
Se
Ph3P
2,2’-bipyridyl
Ph3P
40
14
50
30
40
60
10
5
Supporting information for this article (including experimental and
computational details, spectral data, CV data for compounds 4–7,
and pictures of the crystal packing of 4 and 5) is available on the
2,2’-bipyridyl
[a] Yield of the isolated product.
8814
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 8814 –8818
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